Embryonic stem (ES) cell is a cell isolated from early embryo of mammal, which semipermanently continues to proliferate, while maintaining an ability to differentiate into any cell-in the body, i.e., pluripotency. ES cell was first established in mouse in 1981, and brought an epoch-making technique of gene function analysis using knock out mice. Ever since the establishment of human ES cell was reported in 1998, application thereof to regenerative medicine has been highly expected. It is an attempt to achieve functional recovery by transplanting heart muscle cells or nerve cells differentiated from ES cells into patients with heart infarction and neurodegenerative diseases.
While the cell transplant therapy has been already employed, as typically seen in marrow graft in leukemia, it is associated with two problems of securing sufficient supply of cells to be transplanted and suppression of rejection reaction. Use of the ES cell that divides semipermanently altogether solves the problem of secured supply of sufficient amount of cell. When combined with the somatic cell clone technology, moreover, the rejection reaction can be also overcome. When an ES cell is established from a clone embryo prepared from the somatic cell of a patient and used for transplantation, rejection cannot occur since it has the same gene as does the patient. Therefore, ES cell has the potential to simultaneously solve the two problems in the cell transplant therapy.
While ES cell has the high potential as described above, human ES cell is difficult to establish and maintain as compared to mouse ES cell. Therefore, the development of a reliable establishment technique and a culture technique is necessary. For a human ES cell to be established, moreover, an embryo needs to be sacrificed. When it is combined with the somatic cell clone technology, it easily leads to human cloning. To solve such ethical issues, therefore, the development of a technique to directly produce an ES-like cell having pluripotency from a somatic cell is desired, which does not go through an embryo.
What plays a key role in the development of these techniques is a gene (ES cell associated transcript gene, hereinafter ECAT gene), which is specifically expressed in pluripotent cells such as ES cell and the like. The ECAT gene becomes a marker to determine if the cell is an ES cell. In addition, ES cell can be efficiently selected from a mixed culture of various kinds of cells by combining a control region of ECAT gene that induces ES cell specific expression and a drug resistance gene (JP-T-9-500004; corresponding U.S. Pat. No. 6,146,888). Furthermore, it may be possible to promote conversion of somatic cell to ES-like cell by inducing expression of ECAT gene.
The only one gene reported heretofore as an ECAT gene is transcription factor Oct3 (also called Oct4, POU5f1, hereinafter to be referred to as Oct-3/4) gene. While a similar gene has been reported with regard to human (hereinafter to be referred to as hOct-3/4 gene: Takeda et al., Nucleic Acids Res. 20: 4613-4620, 1992, SEQ ID No; 39), no report has been so far found on verified ES cell specific expression of hOct-3/4 gene. Oct-3/4 is a transcription factor that is specifically expressed in an ES cell and EG cell (embryonic germ cells), whose expression disappears along with the cell differentiation. Therefore, it is used as a marker of ES cell, and efficient establishment of ES cell has been attempted by knocking-in a neomycin resistance gene into its gene locus (JP-T-9-500004; corresponding to U.S. Pat. No. 6,146,888). However, a report has also documented that Oct-3/4 is expressed in trophectoderm cell as well, besides pluripotent cells (Biol Reprod 63: 1698-1705, 2000). Thus, use of Oct-3/4 gene alone as an index results in the selection of cells other than ES cells. To avoid this risk, it is desirable to identify plural ECAT genes and use them in combination.
Even if expression of Oct-3/4 alone in somatic cell is induced, conversion to ES-like cell is not observed. Even if Oct-3/4 is constantly expressed, differentiation of ES cell (differentiation into primitive-endoderm, primitive ectoderm) associated with withdrawal of LIF (leukemia inhibitor factor) cannot be suppressed. To the contrary, an interesting report has been made that, by increasing the expression amount of Oct-3/4 by only about 1.5 times the general level, differentiation similar to that associated with the withdrawal of LIF is induced (Experimental Medicine, 19, 330-338, 2001). As described above, the action of Oct-3/4 is not simple and induction thereof into ES cell by the expression of Oct-3/4 alone in somatic cell is difficult. From this aspect, too, it is considered necessary to combine plural ECAT genes and analyze ES cell.
Nevertheless, ECAT gene other than Oct-3/4 gene has not been found and there is a strong demand for the provision of a new ECAT gene, from the aspects of regenerative medicine and application of ES cells to cell transplantation.